Ultrasound imaging is commonly used for medical diagnostics as it represents a safe and non-invasive technique for real-time imaging. The most commonly used devices at the present time produce 2D images of planes through imaged tissue.
Typically, these images are produced using one or two dimensional arrays of ultrasound sources/receivers which transmit/receive highly directional, short, radio-frequency (typically 3-20 MHz) pulses, with a bandwidth of under one octave.
Individual scans may be one-dimensional (wavefront axial or paraxial) or two-dimensional (wavefield confined to a plane) and use arrays of sources and receivers which are distributed along a line or around a circle. In each case, energy is commonly focussed to target depths within the tissue and the amplitude of reflected energy is detected and used to form the image. Ultrasonic waves may be scattered and modified by a variety of physical processes; however, the term xe2x80x9creflectedxe2x80x9d will be used herein to refer to all ultrasonic waves including all forms of elastic waves reaching the receivers due originally to the ultrasonic sources. Furthermore, the terms xe2x80x9cultrasoundxe2x80x9d and xe2x80x9cultrasonicxe2x80x9d will be used in a more general form than usual herein, to include all forms of high frequency elastic waves, and not just acoustic (sonic, or sound) waves.
Medical practitioners typically wish to study three dimensional structures. A three dimensional effect can be imitated by moving a 2D-ultrasound scanner around interactively and examining continually updated 2D-images. This is unsatisfactory in practice as it is slow compared with the motion of tissue, particularly motion related to the cycle of heart beat and blood flow. Furthermore, the directionality and focussing depth of the 2D images are commonly fixed, often leaving gaps in the reflectivity information.
It would therefore be desirable to provide a 3D imaging system which used true 3D geometry, rather than simply combining information from individual 2D slices. In particular, it would be advantageous to be able to collect and process data for a 3D volume holistically, rather than focussing solely on individual features within the body.
Some 3D imaging systems have been devised which operate by performing a series of 2D scans to provide a 3D record of tissue. These provide additional information to the user; however, image production is slow compared with periodic movements in tissue and typically require several seconds to several minutes to provide images. It would clearly be advantageous to provide an imaging system capable of providing a complete picture in less than the duration of a single human heart beat (say, 1 second).
Furthermore, medical practitioners also wish to view time-varying ultrasound images and, indeed, conventional 2D ultrasound imagers will typically display a rapidly updated image. However, the 3D scanning techniques which compose their image from multiple 2D scans are unsatisfactory for this purpose due to the length of time required. Partly, they are unsatisfactory to watch as the update time is long. In particular, as the update time is long relative to the timescale of periodic tissue movements, accurate time-varying imaging requires use of complex additional techniques such as attempting to strobe the images in phase with the repetitive cyclic motion.
It would therefore be advantageous to provide a system capable of producing time-varying 3D images (4D-images) with an update time faster than the duration of a single heartbeat, preferably much faster.
The following referenced documents, discussed below, contain prior art in the fields of geophysics and pre-stack migration and their disclosure is incorporated herein by reference:
1. Geyer, R. L. (editor), 1989. Vibroseis. Geophysical Reprint Series Number 11. Society of Exploration Geophysicists, Tulsa, Okla., 830 pp.
2. Yilmaz, O., 1987. Seismic Data Processing. Investigations in Geophysics, Volume 2, Society of Exploration Geophysicists, Tulsa, Okla., 526 pp.
3. Geophysics, volumes 1-64, 1936-1999, Society of Exploration Geophysicists, Tulsa, Okla.
4. The Leading Edge (Full title: Geophysics, The Leading Edge of Exploration), volumes 1-18, 1982-1999, Society of Exploration Geophysicists, Tulsa, Okla.
5. Geophysical Prospecting, volumes 1-47, 1953-1999, European Association of Geoscientists and Engineers (formerly European Association of Exploration Geophysicists), Houten, The Netherlands.
6. First Break, volumes 1-17, 1983-1999, European Association of Geoscientists and Engineers (formerly European Association of Exploration Geophysicists), Houten, The Netherlands.
7. Bancroft, J. C., 1997. A practical Understanding of Pre- and Poststack Migrations. Volume 1 (Poststack). Course Notes Series Number 7, Society of Exploration Geophysicists, Tulsa, Okla.
Bancroft, J. C., 1998. A practical Understanding of Pre- and Poststack Migrations. Volume 2 (Prestack). Course Notes Series Number 9, Society of Exploration Geophysicists, Tulsa, Okla.
According to a first aspect of the present invention there is provided a method for elastic wave imaging of a three dimensional object using an array of elastic wave sources and receivers of known position, the method comprising the steps of:
(a) the elastic wave sources emitting elastic wave pulses that are reflected within the volume of the three dimensional object;
(b) the elastic wave receivers measuring the reflected elastic wave pulses;
(c) constructing an image of the three dimensional object from the resulting record of the reflected elastic wave pulses.
characterized in that the elastic wave pulses are ultrasound pulses, the elastic wave sources are ultrasound sources, the elastic wave receivers are ultrasound receivers, the ultrasound receivers measure both the phase and amplitude of the ultrasound pulses, that both phase and amplitude information of the reflected ultrasound pulses is retained and used in constructing an image of the three dimensional object.
An ultrasound pulse may comprise a shot, a shot being a discrete emission of ultrasound from a single ultrasound source.
Preferably, a shot is omnidirectional and point-like in character.
An ultrasound pulse may comprise a plurality of shots activated concurrently with appropriate time delays to produce an approximately planar wavefront moving in a prescribed direction.
Ultrasound pulses may be S-waves.
Typically, each ultrasound receiver records displacement, velocity or acceleration variation as a vector quantity.
Preferably, a plurality of traces are constructed and then used, in an otherwise known method, to construct an image of the three dimensional object, each trace being a record of the data recorded by an individual ultrasound receiver due to an individual ultrasound pulse.
Preferably, an ultrasound pulse is a known encoded signal defined by a time series.
More preferably, different elastic wave pulses are different known encoded signals.
Most preferably, the method includes the step of converting the traces to the form they would have had were each elastic wave pulse in the form of a sharp, short-duration pulse.
Optionally, only low-frequency data might be used for forming an image of the three dimensional object by using a truncated pilot sweep.
An individual ultrasound transducer may act as both an ultrasound source and an ultrasound receiver.
Typically, the position of the sources and receivers is known through their being incorporated onto a fixed, resilient recording surface.
The position and orientation of the recording surface may be monitored throughout data acquisition.
The recording surface may have apertures therein.
The ultrasound sources and ultrasound receivers may be positioned in a regular array.
An array of elastic wave sources and receivers may be separated from the three dimensional object by an ultrasound transmitting medium.
The ultrasound transmitting medium may be a fluid.
Some receivers may be linked together in parallel during data acquisition to form a receiver array.
Preferably, images are calculated in rapid succession to provide a time-varying three-dimensional record.
According to a second aspect of the present invention there is provided elastic wave imaging apparatus for producing an image of a three dimensional object, the apparatus comprising an array of elastic wave sources adapted to emit elastic wave pulses which are reflected within the volume of the three dimensional object, an array of elastic wave receivers for measuring the reflected elastic wave pulses, a dataprocessing means for calculating an image of the three dimensional object; characterized in that the elastic wave pulses are ultrasound pulses, the elastic wave sources are ultrasound sources, the elastic wave receivers are ultrasound receivers, the ultrasound receivers measure both the phase and amplitude of the ultrasound pulses, that both phase and amplitude information of the reflected ultrasound pulses is retained and used in constructing an image of the three dimensional object.
Typically, an ultrasound pulse comprises a shot, a shot being a discrete emission of ultrasound from a single ultrasound source.
Preferably, a shot is omnidirectional and point-like in character.
An ultrasound pulse may comprise a plurality of shots activated concurrently with appropriate time delays to produce an approximately planar wavefront moving in a prescribed direction.
Ultrasound pulses may be S-waves.
Preferably, each ultrasound receiver records displacement, velocity or acceleration variation as a vector quantity.
Preferably, a plurality of traces are constructed and then used, in an otherwise known method, to construct an image of the three dimensional object, each trace being a record of the data recorded by an individual ultrasound receiver due to an individual ultrasound pulse.
Preferably, an ultrasound pulse is a known encoded signal defined by a time series.
More preferably, different elastic wave pulses are different known encoded signals.
Preferably also, traces are converted to the form they would have had were each elastic wave pulse in the form of a sharp, short-duration pulse.
Optionally, only low-frequency data may be used for forming an image of the three dimensional object by using a truncated pilot sweep.
An individual ultrasound transducer may act as both an ultrasound source and an ultrasound receiver.
The position of the sources and receivers may be known through their being incorporated onto a fixed, resilient recording surface.
The position and orientation of the recording surface may be monitored throughout data acquisition.
The recording surface may have apertures therein.
Typically, the ultrasound sources and ultrasound receivers are positioned in a regular array.
Optionally, an array of elastic wave sources and receivers is separated from the three dimensional object by an ultrasound transmitting medium.
The ultrasound transmitting medium may be a fluid.
Some receivers may be linked together in parallel during data acquisition to form a receiver array.
Preferably, images are calculated in rapid succession to provide a time-varying three-dimensional record.